In electromotive machines (such as electric drilling machines, electric spanners, etc), in order to avoid the restriction and inconvenience due to power cord connection, power may be supplied to an electromotive machine from a battery.
There are three kinds of chargers in the prior art. For a charger requiring large power supply, a transformer is used. Since a transformer has a large volume, the charger is heavy and very inconvenient to use. If a charger is designed as an universal charger (which can be used to charge batteries of 7.2 V, 9.6 V, 16 V, etc), matching parts have to be added to the system and consequently the cost is greatly increased. The second kind of charger has a design of electron exchange. This kind of charger needs high cost to manufacture, and is not suitable for general consumers. The third kind of charger has a design of capacitor charging. It is most economic and has a light weight, but in general it often sparks (electric arc) on the terminal as the battery is pulled out. Therefore, the semiconductor device in the charger gets very high current peak which may cause a dangerous accident. Sometimes, even the whole charger may burn out.